This invention relates generally to sewing machines and particularly to a novel warning device that produces a signal to the operator when the supply of the bobbin thread is approaching depletion.
Numerous attempts have been made to develop bobbin thread depletion sensors that are safe, reliable, and can be easily used by both the skilled and the untrained sewing machine operator. Some of the prior devices make use of complex bobbins that are difficult to load such as those containing mechanical linkages in the bobbin that sense the depletion of the bobbin thread supply and then, for example, trigger electrical signalling circuits. For example, in the U.S. Pat. No. 3,129,680, the depletion of the bobbin thread supply releases a spring loaded magnet from a position adjacent the hub of the bobbin and permits the magnet to reposition itself along the periphery of the bobbin where it can actuate a magnetic reed switch that signals the operator of the bobbin thread supply deletion.
In another system, the depletion of the thread releases a spring loaded conductive member which bridges a pair of electrical contacts to complete the electrical signalling circuit. While such systems may prove to be reliable, the complexity of the bobbin structure is both costly and confusing to the operator. Furthermore, signalling systems employing electrical signal contacts may prove hazardous to even the highly skilled operator.